The miniaturization of components has become a major focal point of semiconductor technology in recent decades because of the possibility of achieving integration densities as well as high switching speeds. Increasing difficulties in light-optical photo techniques are encountered when the size of such components falls below 1.mu.m. Self-aligning processes wherein the gate metallization, for example in MESFED, acts as mask with reference to subsequent process steps (for example n.sup.+ -implantations and/or source/drain contacts) have been employed with some success.
In such self-aligning processes, further tempering steps are usually executed after the application of the gate metallization, requiring that the metallization be adequately stable with respect to such tempering processes. In order to achieve this stability along with electrically effective gate lengths below 1 .mu.m, low lead resistances over adequately large gate cross-sections must be realized. To this end, the metal contacts are fashioned with overhanging edges.
Gate metallizations having a T-shaped structure have been produced using a dry etching process. At least two layers composed of different metals are required for the production of such overhanging metallizatons. Photoresist or a third metal structure deposited in a lift-off technique can serve as etching mask. The selection of the metals in such a process is critical because of the selective etching that is required. The first metal layer deposited must be attached more strongly during etching than the second metal layer situated thereabove. The resultant gate metallizations have a first metal layer and a second metal layer that form a step-shaped discontinuous cross-section. Such a structure is commonly produced in that, first, an etching mask is applied and the two metal layers are then etched down on a straight line to the semiconductor surface with an anisotropic etching. High anisotropy is attained in the etching in that the pressure of the plasma is kept low during the dry etching process and the cathode voltage is set high. After the two metal layers have been etched out they are of the same diameter and extend perpendicular to the surface of the semiconductor layer structure. Next, a considerably intensified etching attack is performed on the second metal layer. This is achieved, for example, by changing the gas composition and/or by increasing the process pressure. As a rule, this is carried out with a surface-preserving, isotropic etching process that sequences with a low power density. The size of the underetching is thereby determined via the etching time. There is no end point signal as can be detected, for example, when etching through from a defined layer to a different layer (e.g., where detection is via modification of the optical properties of the plasma). This limits the precision and reproducibility of the underetching in practice. The cross-section of the metal contact structure thus acquires a "T-shape"
When etching is performed using an RIE system, what is referred to as "positive column", in which the potential is constant, forms in front of the anode. This positive column extends toward the surface of the cathode. A dark field zone is situated at the surface of the cathode, this dark field zone being separated from the positive column by an equipotential surface that proceeds nearly parallel to the surface of the cathode and of the substrate.